Fungicide-resistant frogeye leaf spot in Arkansas soybeans

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click image to zoomFigure 1. Soybean leaflet with immature and developed FLS lesions. Recently, there have been a few reports of frogeye leaf spot (FLS) showing up on susceptible soybean varieties in a couple areas the state. Disease severity has ranged from a trace to very light (Fig. 1). Given that some will begin to consider fungicides options in the near future it seems appropriate to summarize the occurrence of new fungal strains that were found last year in Arkansas. Also, there is a little update on the status of SBR in the region at the end of this report.

Figure 2. Counties (blue) where strobilurin-resistant strains of FLS were collected and confirmed in 2012. Soybean growers in the state have relied on fungicides like Headline, Quadris, Evito, Gem, etc. to control FLS. All these fungicides contain the same mode of action and collectively referred to as strobilurin fungicides. In 2012, a new strain of the fungus that is resistant to all strobilurin fungicides was confirmed in nine Arkansas counties (Fig. 2, Clay, Lawrence, Jackson, Poinsett, St. Francis, Phillips, Desha, Drew, and Chicot). Therefore, selecting the right fungicide chemistry will be an important consideration when managing FLS in 2013.

A strobilurin-alone program will not effectively manage FLS where the new strain was confirmed or where strobilurin failures were observed in 2012. A significant population of these new fungal strains will likely occur in new areas where soybeans are continuously cropped and a strobilurin-alone program has been used to control FLS and other fungal diseases. These new fungal strains can be managed with triazoles fungicides (alone or as a mixture) or resistant soybean varieties. Additionally, tillage and one year crop rotation can reduce the overwinter pathogen survival on crop residue. Triazole-resistance to fungi similar to FLS is not new therefore, applying foliar fungicides only to control plant diseases will prolong their usefulness in row crop production.

Recently, Soybean rust was reported on soybean in two fields near Alexandria, La. To date, this is the closest SBR has occurred near Arkansas (~ 120 miles from the Arkansas border); however, the disease incidence was low, so the current risk to Arkansas growers is very low. SBR is likely to move up the MS River as it has done in the past and will probably occur in MS before it is found in Arkansas. Future updates will be posted on this blog as well as the ipmPIPE (http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi/), another good resource for SBR updates.  There is no recommendation for treatment for SBR at this time, as there is no SBR in the state.


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